The elevator industry traditionally has provided some form of annunciator or "sounder" function in conjunction with each landing serviced by an elevator car. Early versions of these annuciator devices were electro-mechanical in nature, a bell being actuated to indicate arrival of the car at a given floor station. These electro-mechanical devices were prone to break-down and eventually have been phased out in favor of electronic approaches to the function.
Loudspeakers of conventional design have been suggested for employment in each car and at each floor landing to announce arrivals. However, disadvantages are attendant with their use. The loudspeakers are driven by an oscillator in a manner mimicking a chime. For example, one chiming sound may represent that the elevator is progressing in an upwardly direction, while two chimes would indicate that the elevator is downwardly progressing. Similarly, a sequence of singular chimes may indicate that the door is being held open for an unduly lengthy period of time.
Because of the broad frequency response and the low resonance characteristics of loudspeakers, they readily have been capable of producing quality chime sounds. However, because loudspeakers are formed of paper cones and the like, they are prone to damage in the environment of elevators, which is more rigorous than may be expected. For example, vandalism, moisture, and excessive dirt when combined with the very dynamic conditions extant at the elevator shaft will cause their breakdown at an unacceptably high level.
Recently, the industry has turned to the use of piezoelectric crystal driven sounders having the promise of much improved durability when employed in this elevator environment. In using these devices, however, several undesirable characteristics of their operation had to be overcome. In this regard, the piezoelectric driven devices are highly resonant during several modes of their performance, depending upon the wave pattern of the diaphragms to which they are attached. Thus, the development of a desired chime sounding, as for example a "gong" becomes difficult to achieve. A gong-like sound requires a decaying amplitude drive at lower frequencies which have proven difficult to develop. However, improved lower frequency outputs have been achieved through resort to dual chamber sound box structures designed following Helmholz theory.
When employed in the environment of the elevator shaft, however, these devices have been prone to react to unusual spurious electrical phenomena to falsely actuate. Attempts at isolating the causes of such spurious phenomena have not been particularly successful, perhaps due to the extreme length of typical elevator shafts and the related dynamics of the elevator within that shaft environment.